The present invention relates to a curtain coater and to a curtain-coating method.
In a curtain coater, the coating mix is applied to the surface of a moving web of paper or board, generally from a nozzle extending over the full cross-machine width of the web and located above the web being coated, whereby the coating mix can fall onto the web surface as curtain-like shower. Curtain coating is categorized as a noncontacting coating method, wherein the applicator itself makes no contact with the web being coated, but instead, the coating mix is applied to the web surface in the form of a free-falling curtain of coating mix. The technique of curtain coating is described, e.g., in publication DE 196 22 080.
During its travel, a moving web gathers a thin boundary layer of air that moves along with the web. In curtain coaters, the momentum of the coating mix applied to the web surface is small as compared to the momentum of the coating mix amount directed from a jet applicator, for instance, which means that the boundary air layer traveling on the web surface can easily scatter the curtain of coating mix flowing from the nozzle of a curtain coater thus making the applied coating layer uneven. With higher web speeds in the coater station, the problem is accentuated due to the faster speed of the boundary air layer and its higher momentum. Hence, the control of the boundary air layer behavior at higher web speeds becomes one of the most significant factors affecting the runnability of a curtain coater.
The problem associated with the boundary air layer can be diminished by way of, e.g., making the height of the falling curtain of coating mix larger thereby increasing its falling velocity or by increasing the amount of coating being applied, whereby the momentum of the coating mix curtain is increased and the falling curtain can more readily penetrate through the boundary air layer traveling on the web surface. However, it is generally not possible to make the falling height of the coating mix curtain sufficiently large because the coating mix curtain begins to converge and separate into streamlets with a larger falling height. Moreover, the increase of the amount of the applied coating mix necessitates doctoring away the excess coating from the web surface.
It is an object of the present invention to provide an entirely novel type of curtain coater and curtain-coating method offering an essential improvement in the reduction of the amount of boundary air penetration to the application zone of a curtain coater.
The goal of the invention is attained by way of placing a doctoring means upstream in front of the application point in the travel direction of the web being coated, the device serving to remove the boundary air layer from the surface of the traveling web. The purpose of the doctoring means is to bring about a significant reduction in the amount of the entrained air traveling along with the web to the application zone. In one embodiment of the invention, the amount of the boundary air coming to the application zone is reduced by means of a suction nozzle cooperating with the air-doctoring element, whereby the boundary air layer is removed via the suction nozzle by a vacuum. Additionally, the adherence of the coating mix curtain to the web surface can be augmented by means of a gas-injection nozzle mounted downstream after the applicator nozzle in the travel direction of the web, whereby a gas jet can be directed from the gas-injection nozzle toward the coating mix curtain. Hereby, the combined momentum of the coating mix curtain and the gas jet becomes sufficiently energetic to force the coating mix to penetrate through the boundary air layer traveling on the web surface.
The invention offers significant benefits.
In a curtain coater according to the invention, the amount of boundary air traveling on the web being coated to the application zone can be reduced significantly as compared with conventional curtain coaters, whereby the coat quality and web runnability in the coater are improved. The web speed in a curtain coater according to the invention can be readily increased because the boundary air layer can be removed effectively from the surface of the running web prior to application.